1. Field of the Invention
The present invention relates to an optical sampling apparatus for measuring a time waveform of signal light with high precision, and an optical sampling oscilloscope forming part of the optical sampling apparatus.
2. Description of the Related Art
Conventionally, a digital (sampling) oscilloscope has become commercially practical and widespread as an apparatus for observing the time waveform of a transmission signal.
In the digital oscilloscope, the signal intensity of the electric signal which changes in intensity with time is detected at predetermined time intervals, and signal processing for joining in time the signal intensity of the detected electric signal is performed, and the joined time waveform is, for example, displayed on the display screen. Since these processes are electrically performed, the time resolution for detection of the signal intensity of a transmission signal is restricted by the operation speed of electronics.
Furthermore, when the time waveform of signal light of an optical fiber communications system, etc. is observed, a photoreceiver is further included for converting the signal light into an electric signal, and the above-mentioned process is performed on the converted electric signal. In this case, the time resolution for detection of the signal intensity is restricted by the operation speed of the electronics.
Recently, technologies for realizing superfast optical communications have made remarkable progress. At present, a system capable of performing a system for performing superfast optical communications with the transmission speed of 160 Gbit/s has been developed using an ultrashort optical pulse in or within 1 picosecond.
In developing the above-mentioned superfast optical communications system, for example, when the performance of the system is verified, it is necessary to prepare an apparatus capable of observing the time waveform of signal light with high precision.
Using the above-mentioned oscilloscope, the processes of electrical time sharing, detection of signal intensity, etc. are performed after temporarily converting signal light into an electric signal. Therefore, satisfactory time resolution cannot be obtained in the time waveform observation of not only signal light having the transmission speed of 160 Gbit/s, but also signal light having the transmission signal of 10˜40 Gbit/s. As a result, a time resolution limit is proposed.
Thus, an optical sampling system for optically sampling signal light has been developed as the technology of overcoming the time resolution limit caused by the operation speed limit of the electronics, and an optical sampling oscilloscope which adopts the above-mentioned system is being commercially practical.
The optical sampling oscilloscope inputs signal light having an iterative frequency and sampling pulse light having an iterative frequency a little different by an integral submultiple of the iterative frequency (several 100 HZ or some kHz) to a nonlinear optical crystal, and detects the optical intensity of the sum (or difference) frequency light output under the second-order nonlinear optical effect from the nonlinear optical crystal. At this time, the pulse width of the sampling pulse light is generated such that it can be sufficiently smaller than the pulse width of the signal light and can be synchronous with the pulse width of the signal light, thereby receiving the sum (or difference) frequency light and observing the time waveform having the time resolution of picosecond (for example, refer to document 1).                [document 1] Japanese Patent Laid-open Publication No. 2003-65857 (paragraph “0020”-“0022”, FIG. 2).        
However, the conventional sampling oscilloscope has been configured such that, as described above, the time waveform of an optical signal can be detected by receiving the sum (or difference) frequency light the sum (or difference) frequency light caused by the second-order nonlinear optical effect of the nonlinear optical crystal. However, the generation efficiency of the sum (or difference) frequency light generated by the second-order nonlinear optical effect is very small.
Therefore, there has the problem that a very strong power of sampling pulse light has to be input to the nonlinear optical crystal so as to generate the sum (or difference) frequency light having sufficiently high intensity to detect the time waveform of signal light.
Additionally, since the S/N ratio (signal-to-noise ratio) of the sum (or difference) frequency light generated through a nonlinear optical crystal is considerably degraded, the noise is amplified when a signal is amplified based on the sum (or difference) frequency light, thereby failing in detecting the time waveform of the signal light with high precision.